1. Field of the Invention
The present invention relates to a gas sensor for detecting a concentration of a specific gas contained in a measurement gas.
2. Description of Related Art
There are known various gas sensors which can be mounted on an exhaust system of an internal engine of a vehicle for measuring a concentration of a specific gas such as oxygen or nitrogen oxide contained in an exhaust gas as a measurement gas. Such gas sensors includes ones that include a sensor element for detecting a concentration of a specific gas contained in a measurement gas, a first insulator through which the sensor element is inserted to be held therein, a second insulator disposed on the proximal side of the first insulator so as to cover the proximal side of the sensor element, and a contact member held by the second insulator and being in contact with electrode terminals of the sensor element. For example, refer to Japanese Patent Application Laid-open No. 2003-43004.
The gas sensor described in this patent document includes a positioning structure provided in the end surface on the distal side of each of the first and second insulators for positioning the first and second insulators. The positioning structure of this gas sensor is configured to restrict movements of the first and second insulators in the direction perpendicular to the direction in which the sensor element is resiliently restrained or held by an electrode spring serving as the contact member. Restricting movements of the first and second insulators makes it possible to fix the positional relationship between the sensor element and the electrode spring in this direction to thereby provide a reliable electrical continuity between them.
However, in the gas sensor described in the above patent document, movements of the first and second insulators in the direction in which the sensor element is resiliently held are not restricted. Accordingly, the sensor element may move in the direction in which the sensor element is resiliently held by the electrode spring at the time of assembling the gas sensor, for example.
Therefore, it may occur that electrical continuity between the sensor element and the electrode spring becomes insufficient due to sensor-to-sensor variation in the spring load applied to the sensor element. Meanwhile, there is a strong demand to make such a gas sensor compact in size for the purpose of improving its performance (early activation, for example) and reducing its manufacturing cost. As the size of the gas sensor becomes smaller, the required assembly accuracy becomes higher, and accordingly the above problem becomes more pronounced.